This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The geomagnetic field is an omnipresent source of information for magneto sensitive organisms. Birds, fish, and other animals are known to detect the direction and intensity of the local magnetic field. There are two hypotheses for the primary process underlying magnetic sensitivity;one involves magnetite and the other a magnetically sensitive photochemical reaction. In the second one, light induces an electron transfer from a photoreceptor in a radical pair sensitive to external magnetic filed. Very recently, the flavin photoreceptor cryptochrome has emerged as a promising photo-magnetoreceptor candidate. Based on their known photocycling, there are several putative magnetically sensitive reactions of the cryptochrome. The first of these reactions involves the excitation of the flavin by light to form its excited state which takes an electron to a nearby tryptophan to initiate a chain of intramolecular electron transfer to the surface of the protein. Modeling suggests that magnetic field effect occurs on this reaction cascade. The objective of the experiments is to determine whether the purified protein is directly sensitive to applied magnetic fields. Therefore, lights and magnetic field are applied and the photocycle of the cryptochrome is followed directly using the spectrophotometer and fluorometer available in the Laboratory for Fluorescence Dynamics.